Fix providing an explicit branch/ref to land-via-bot
Fix providing an explicit branch/ref to land-via-bot

Summary: The wildcard argument gives an array, not one object.

Test Plan:
Pre-patch, this gives a backtrace:
`arc land-via-bot master --hold`
Post-patch, this correctly identifies the branch and associated revision:
```
arc patch <this-patch>
arc land-via-bot HEAD --hold
```

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6969

Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc